Immiscibility of components of commingled plastics waste recyclates is the greatest barrier to achieving its utilizable application properties. The immiscibility of different polymers itself, caused by thermodynamic limitations, would not be a hindrance to the preparation of materials with practically utilizable properties if the condition of their mutual compatibility were fulfilled. (Compatibility is generally understood as the ability of immiscible polymers to form such supramolecular structure of the resulting blend that enables achieving material properties close to or better than those of polymer components of the blend.) However, mutual compatibility of most thermoplastic polymers is very low. From mutual immiscibility and low interphase adhesion result strong separation trends of the blend components. All material properties dependent on stress transfer (tensile strength, ductility, toughness) in such blends remain deep below the level corresponding to the additivity rule. The materials thus characterized are practically unusable.
Separation tendencies of components of blends of mutually immiscible polymers can be efficiently suppressed by their compatibilization. As efficient compatibilizers of polyolefin blends proved successful statistical and block ethylene—propylene copolymers (e.g., according to DE 28 49 114, U.S. Pat. No. 4,319,005 or U.S. Pat. No. 4,567,847), and styrene—butadiene or hydrogenated styrene—butadiene block copolymers or their blends with ethylene—propylene copolymers for compatibilization of polyolefin—polystyrene mixtures (e.g., according to Czech Application PV 2000-525). In addition to the additive compatibilizers, also reactive systems based on initiated radical reactions of polyolefin components were successfully tested (e.g., according to Czech patents CZ 284819 and CZ 284862).
Used plastic packings and multimaterial products of short and medium lifetimes from households and small firms are the largest source of plastics waste. The commingled plastics waste consists of approximately 65% polyolefins (low-density and high-density polyethylene, polypropylene), 11% polystyrene plastics, 13% polyester plastics (mainly poly(ethylene terephthalate)) and small proportions of poly(vinyl chloride) plastics and polyamides. Plastics waste coming from municipal scrap is usually unsorted or with removed, usually incompletely, poly(ethylene terephthalate) bottles. Polymer components of the waste are devalued to various extent by thermal and weathering degradation and, in addition, the waste contains other contaminants of various origin. The strength characteristics of blends of thermoplastics damaged by degradation are even worse than with those undamaged by degradation. Sorting and cleaning of the blends is a technologically complex and energetically demanding process. For processing of commingled plastics waste, a special technology developed for the purpose is often used, based on mixing a commingled plastics melt in an extruder-and-immediate extrusion of the melt into a mold. The advantage of this method of processing of commingled plastics waste consists in that comparatively large articles can be easily obtained. A drawback is not quite good mechanical properties of the final recyclate, which can hence compete in applications only with cheap sorts of wood or concrete. This recycling method is suitable for production of massive articles fulfilling only low aesthetic and strength demands. The economical balance of the recycling method for commingled plastics waste is not very good and is usually close to the cost-effectiveness limit.
In the study of structure and properties of commingled thermoplastic waste, it was surprisingly found out that an admixture of an ethylene—propylene copolymer or styrene—butadiene block copolymer in combination with secondary aromatic amines leads, after subsequent processing of the blend by melt mixing, to considerable enhancement of toughness of the resulting material. Further it was found out that an admixture of an ethylene—propylene copolymer or styrene—butadiene block copolymer in combination with secondary aromatic amines to mixed degradation-damaged plastics containing polyolefins and polystyrene exhibits a synergistic compatibilization effect, i.e., the toughness of the resulting material is considerably higher than that of the same mixed degradation-damaged plastics compatibilized with an admixture of an ethylene—propylene copolymer alone or styrene—butadiene block copolymer alone or with a combination of an ethylene—propylene copolymer and styrene—butadiene block copolymer. The compatibilization efficiency of the method according to the invention is noticeably higher with degradation-damaged commingled plastics waste containing polyolefins and styrene plastics, where the hitherto used compatibilization methods practically fail.
The recyclate of commingled plastics waste compatibilized by the method according to the invention can then show high toughness while maintaining a balanced complex of the other utility values.